Light modulating displays use arrays of light modulating elements to modulate light to produce visual images. Examples of such displays include displays as described in PCT Patent Publications PCT/CA03/00350 and PCT/CA2005/001111, and in United States patent applications US 2007/0268577 A1, US 2008/0043034 A1, US 2008/0043303 A1, US 2008/0111502 A1 and US 2008/0074060 A1, all of which are hereby incorporated herein by reference for all purposes.
Light modulating displays may be used to show video content. Because cathode ray tube (CRT) displays were the original video display technology, video content is typically provided to displays serially for raster scan display. In CRT raster scanning, an electron beam sweeps horizontally left-to-right across a screen at a steady rate, then turns off and moves rapidly back to the left, where it turns back on and sweeps out the next line. After the end of the last line of the image, the electron beam turns off and moves rapidly to the top-left to begin scanning the next image. Signals driving raster scanning displays typically include an image information signal, a horizontal synchronization signal and a vertical synchronization signal. The horizontal synchronization signal provides a pulse timed to coincide with the end of a horizontal line of element-specific image information. This pulse is used in CRT displays to control the turning off of the electron beam between horizontal lines. The vertical synchronization signal provides a pulse timed to coincide with the end of the last line of the image. This pulse is used in CRT displays to control the turning off of the electron beam between images.
Light modulating displays, such as LCD displays, typically accommodate video content provided serially for raster scan display by writing data for the next image to be displayed into a buffer. At some time after data for the next image to be displayed has been written into the buffer, the data in the buffer is latched into driving circuits for light modulating elements of a spatial light modulator (SLM), thereby replacing the existing data driving the light modulating elements with new data. The light modulating elements are then driven according to the new data.
As used herein in connection with light modulating elements and SLMs, the term “updating” includes driving a light modulating element according to new data. Some light modulating displays update the SLM in stages. At each stage image data is latched into the driving circuits of a plurality of light modulating elements.
Light modulating elements typically take some time to switch from one state to another. When the driving values of light modulating elements, such as pixels of LCD modulators, are changed, the light modulating elements may overshoot the desired output level and oscillate about the desired level before settling to the correct level. Such response characteristics can lead to an undesirable visual characteristic known as “inverse ghosting”.
In displays known as dual modulation displays, both the SLM and a light source may be updated according to new image data. Some such dual modulation displays are known to be capable of delivering enhanced dynamic range and improved contrast. In dual modulation displays the SLM may be updated according to new image data before, after or at the same time as the light source is updated. The inventors have determined that timing of the updates according to new image data of the SLM and the light source can affect the visual characteristics of images, and produce perceived negative phenomena such as flicker and motion blur artifacts.
In some applications, dual modulation displays are used to display video. A display showing video typically receives image data representative of images (frames) that are to be displayed in sequence. It is frequently desirable to update both the SLM and the light source to display new frames. Updating both a SLM and a light source can be computationally intensive. Some displays lack the computational resources to update both the entire SLM and the entire light source at the video frame rate.
There is a need for methods and apparatus for controlling dual modulation displays in ways that reduce or avoid undesirable visual artifacts. Preferably, such apparatus and methods permit scaling or managing the computational burden associated with updating a light source. There is a specific need for such methods and apparatus for controlling light sources of liquid crystal displays.